One of the techniques for calculating a change of the ground surface or an object is a technique called PS-InSAR (Permanent/Persistent Scatters Interferometric Synthetic Aperture Radar) (e.g. Non-Patent Document 1 and Non-Patent Document 2). PS-InSAR is a technique for measuring displacement at a point on the ground surface or a certain object by applying interferometry to SAR data which is data obtained with respect to a point having PS (Permanent/Persistent Scatters) properties by a synthetic aperture radar (SAR). Radio waves have a characteristic of allowing their observation irrespective of weather or even at night because they pass through clouds or rain unlike light waves. In the following, a point on the ground surface or on an object whose displacement is to be measured in PS-InSAR will be referred to as a measuring point. Each point on the ground surface or on an object corresponds to each resolution cell in a field of vision for image capturing of the synthetic aperture radar.
Here, PS properties are properties in which a radio wave scattering characteristic is not changed with a lapse of time. While plants or waves are among those not having PS properties, most of artificial structures have PS properties at numbers of places thereof. In PS-InSAR, however, use of a certain place as a measuring point requires a certain degree of backscattering intensity (hereinafter referred to as reflection intensity) in addition to PS properties. This is because when a reflection intensity is low, signals may be buried in noise. Such a limitation on measuring points decreases the advantage of PS-InSAR.
It is known that for example, at a point having such a shape as a single flat board, backscattering is so weak that even if it has PS properties, the point is less likely to be considered as a measuring point. It is also known that for example, at a point having such a shape made of three square or rectangular equilateral triangle plates joined together so as to be opposed to each other at right angles as used in a corner reflector (CR) (e.g. four corners of a window frame), backscattering is easily returned, so that such a point is appropriate as a measuring point.
As a technique related to the present invention, for example, Patent Document 1 recites a technique of storing, in a storage device in advance, a result as polarization property data for each measuring position, which result is obtained by measuring at least three kinds of polarization components among scattered waves obtained from a plurality of reflected radio waves having different polarization properties and determining whether an object at each measuring position is an artifact or not using the stored polarization property data.
For example, Non-Patent Document 3 recites that pseudo color composite display of full polarimetric SAR image data is executed, with a set of three polarizations (e.g. HH polarization, HV polarization and VV polarization) associated with red, green and blue, each set having four kinds of data whose values are different from each other. The four kinds of data are assumed to be backscattering coefficients of full polarimetric SAR image data, including an ellipse orientation angle ψr of a reception polarization, an ellipticity angle χr of a reception polarization, an ellipse orientation angle ψt of a transmission polarization and an ellipticity angle χr of a transmission polarization. Non-Patent Document 3 also discloses, as one of application examples of full polarimetric SAR image data, an example where a transmission and reception condition that attains the highest contrast between two target objects is obtained and based on the condition, a specific target object to be identified and another target object are displayed such that a ratio of a pixel value of the specific target to that of another target is the highest.